regen

* regenerate

* using latest protoc-gen-go

should be fixed in https://github.com/grpc/grpc-go/pull/2242#pullrequestreview-140912658 (#86)

* disable go1.6

gRPC imports golang.org/x/sys/unix which currently does not build on
go1.6. Depending on how compatibility story plays out, we'll either drop
1.6 support or add back 1.6 CI.

This commit also slightly changes the CI version "structure".
We previously list all Go versions to test against. This commit
makes us test against 1.7 and the latest version.
This means we don't have to manually bump version when new releases come
out. We have to be doing something really strange if we build against
1.7 and 1.10, but not 1.8 and 1.9.

This reverts commit c19ce610.

Some .proto files are being moved from googleapis to api-common-proto.
However, one file will exist in both places for a while.

regen.go is modified to deduplicate files by name,
and regen.sh is modified to use api-common-protos in addition to
googleapis.

This update uses the latest version of the protoc Go plugin, which
reverted a change in UTF-8 validation for proto2 (and added fields
for proto3). This change removed validation of UTF-8 in proto2
and added a new field in proto3 that signals validation.

Relevant PR: https://github.com/golang/protobuf/pull/628

protoc-gen-go: 668a607657a5d387d6333648a2d9c749761fdc69
googleapis: c89b6330b6386298f8ea65e47e0b77b28294e3e7

googleapis: da6951a795d67ba3feb0ebf51d6da79919b626a9

googleapis: aa14ef295443c528d6c02aa4ddb5998a09223c69

googleapis: b12480746ea88ceb84bbdd1df336deef831dd9bb

googleapis: 26b634d2724ac5dd30ae0b0cbfb01f07f2e4050e